Microfluidic device and method using double anodic bonding

ABSTRACT

A microfluidic device for use with a microfluidic delivery system, such as an organic vapor jet printing device, includes a glass layer that is directly bonded to a microfabricated die and a metal plate via a double anodic bond. The double anodic bond is formed by forming a first anodic bond at an interface of the microfabricated die and the glass layer, and forming a second anodic bond at an interface of the metal plate and the glass layer, where the second anodic bond is formed using a voltage that is lower than the voltage used to form the first anodic bond. The second anodic bond is formed with the polarity of the voltage reversed with respect to the glass layer and the formation of the first anodic bond. The metal plate includes attachment features that allow removal of the microfluidic device from a fixture.

TECHNICAL FIELD

The invention relates to anodic bonding and structures and methods ofmaking microfluidic devices such as are used in organic vapor jetprinting systems.

BACKGROUND OF THE INVENTION

Various techniques have been developed for depositing and/or patterningorganic materials on a substrate for use in constructing organicopto-electronic devices such as organic light emitting diodes (OLEDs),organic phototransistors, organic photovoltaic (PV) cells, or organicphotodetectors. These techniques include vacuum thermal evaporation,solution processing, and organic vapor phase deposition, along withprinting techniques such as inkjet printing, nozzle printing, thermalvapor jet printing, and organic vapor jet printing (OVJP). Some of thesetechniques include flowing high temperature fluids and/or high pressurefluids through various components and interfaces between components thatmay range in size from a macroscopic level, at which bulk materials aretypically provided and stored, to a microscopic level, at which thematerials may be effectively utilized. One or more seals may be providedat these interfaces to prevent fluid leakage.

Print heads for organic vapor jet printers sometimes require replacementdue to wear over time, damage, or some other reason. Such print headsare typically attached and sealed directly to a fixture using anadhesive, where the fixture is the source of high temperature gases usedin the printing process. The adhesive may be an epoxy or similaradhesive capable of maintaining bond strength at high temperatures. Inorder to replace a print head that is adhesively attached, the adhesivebond must be broken. This often requires chiseling or otherwisedestructively removing the adhesive and/or print head from the fixture,followed by sand-blasting or other abrasive cleaning of the fixturemounting surface in preparation for another print head attachment.

SUMMARY

According to one aspect of the invention, a microfluidic device isprovided for receiving fluids from a fixture. The microfluidic deviceincludes a metal plate having a surface that includes a fluid outletport and a glass layer having an inlet side and an opposite outlet side.The glass layer is bonded on the inlet side to the surface of the metalplate and has a fluid passage interconnecting the inlet and outlet sidesof the glass layer to allow fluid flow therethrough. At least a portionof the glass layer fluid passage is aligned with at least a portion ofthe fluid outlet port of the metal plate, whereby pressurized fluidexiting the fluid outlet port of the metal plate can enter the fluidpassage and can be communicated to the outlet side of the glass layer.The microfluidic device also includes a microfabricated die having aninlet side. The die is bonded on the inlet side to the outlet side ofthe glass layer and includes a fluid passage to allow fluid flowtherethrough. At least a portion of the fluid passage of the die isaligned with at least a portion of the fluid passage of the glass layer,whereby the pressurized fluid communicated to the outlet side of theglass layer can enter the fluid passage of the die. The glass layer isdirectly bonded to the metal plate via an anodic bond. In someembodiments, the glass layer can also be directly bonded to the die viaa second anodic bond such that the microfluidic device includes a doubleanodic bond.

According to another aspect of the invention, an organic vapor jetprinting device is provided and includes a fixture having fluid outletports for supplying high temperature gases under pressure and a metalplate fastened to the fixture. The metal plate includes fluid inletports, each of which is aligned with one of the fluid outlet ports ofthe fixture to receive the high temperature gases. The printing devicealso includes a seal located at an interface of the fixture outlet portsand the plate inlet ports to prevent leakage of the high temperaturegases out of the printing device. The printing device additionallyincludes a silicon-based die bonded to the metal plate and having fluidinlet ports and nozzles that are in fluidic communication with the fluidinlet ports of the die. The metal plate also includes fluid outlet portsthat are in fluidic communication with the inlet ports of the metalplate and that are in fluidic communication with the fluid inlet portsof the die so that the high temperature gases can be conducted from thefixture, through the seal, through the metal plate and to the nozzles ofthe silicon-based die.

In accordance with another aspect of the invention, a method is providedfor forming a double anodic bond. The method includes the steps of: (a)stacking a glass plate together with one of: a silicon-based plate and ametal plate, so that planar surfaces of each plate contact each other atan interface; (b) forming an anodic bond at the interface by applying avoltage across the stacked plates; (c) stacking the other one of thesilicon-based plate and the metal plate together with the bonded platesso that the glass plate is interposed between the silicon-based plateand the metal plate; and (d) applying a voltage across the stackedmetal, glass, and silicon-based plates to form the double anodic bond,wherein the polarity of the voltage is reversed from the voltage appliedin step (b) with respect to the plates bonded in step (b).

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred exemplary embodiments of the invention will hereinafter bedescribed in conjunction with the appended drawings, wherein likedesignations denote like elements, and wherein:

FIG. 1 is a partially exploded view of portions of an organic vapor jetprinting (OVJP) assembly, according to one embodiment, showingmicrofluidic print head components separated from a fixture that cansupply high temperature gases to the device;

FIG. 2 is a cross-sectional view of an assembled portion of the OVJPassembly of FIG. 1, including a microfluidic print head that isremovably fastened to a fixture, according to one embodiment;

FIG. 3 is a cross-sectional view of a glass plate and a silicon-basedplate undergoing an anodic bonded process, according to one embodiment;

FIG. 4 is a cross-sectional view of the plates of FIG. 3 undergoing adouble anodic bonding process, according to one embodiment;

FIG. 5 is an atomic force micrograph of a bonding surface of a metalplate after preparation for anodic bonding;

FIG. 6 is an atomic force micrograph of a surface of a glass plate afteran opposite surface of the glass plate has been anodically bonded to asilicon-based plate; and

FIG. 7 is an atomic force micrograph of the surface of the glass plateof FIG. 6 shown after an RCA cleaning process.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT(S)

The illustrated embodiment is directed to an organic vapor jet printing(OVJP) device as but one example of a microfluidic delivery system thatmay benefit from the teachings herein. Utilizing the below-describedconfigurations and methods, microfluidic devices may be constructed tobe easily replaceable or interchangeable so that, for example, worndevices such as print heads can be easily replaced or so that differentfluids from the same fluid supply or source can be mixed in differentways to deliver different mixtures of fluids for microdeposition onto asubstrate or other component according to one or more fluid circuitsbuilt into the device. This flexibility is made possible in part by thedevelopment of a double anodic bonding technique that can provideexcellent interfacial seals on opposite sides of a glass or other layerof the device. It is noted that the appended drawings are notnecessarily to scale and that any cross-hatching shown is provided forclarity in distinguishing among different components and is not meant tolimit the types of materials that may be used with each component.

Referring to FIG. 1, a microfluidic delivery system is shown in the formof an OVJP printing device 10. The illustrated device 10 includes afixture 12 and a microfluidic device 14 (shown in exploded view) forattachment to the fixture. Microfluidic device 14 is a microfluidicprint head in the illustrated embodiment. Fixture 12 may be provided inany number of forms or configurations and may generally be described asany component that provides one or more fluids to microfluidic device 14when it is attached to the fixture. In the illustrated embodiment,fixture 12 includes a mounting surface 16, having fluid outlet ports 18and attachment features 20 formed therein, along with various conduits22 and ports 24 for transporting and delivering high temperature and/orhigh pressure gases or other fluids to the attached microfluidic device14. For example, organic materials supplied to the conduits 22, and theconduits 22 may be heated to vaporize the organic materials. The same ordifferent organic materials can be disposed in each individual conduit22. In one example, a semiconductor host material may be disposed in oneconduit and a semiconductor dopant may be disposed in a differentconduit. Ports 24 may be used to introduce a carrier gas, such asnitrogen or other gas that does not react with the organic materials, tothe printing assembly 10. The carrier gas flows through conduits 22 tomix with the vapor produced therein and to deliver the organic vapor tothe print head 14. In the illustrated embodiment, the fixture 12delivers high temperature gases to print head 14 through outlet ports18. Fixture 12 may of course include any number of other components notshown or described here. A more detailed description of exemplary OVJPdevices, components, and the operation thereof can be found in U.S.Patent Application Publication Nos. 2010/0245479A1 and 2010/0247766A1,the complete contents of which are hereby incorporated by reference.

Print head 14 is a component located at an end of the OVJP assembly 10that receives one or more fluid(s) from fixture 12 at one end, anddelivers the fluid(s) at an opposite end through openings or channelsthat are generally on a micron scale (e.g., from about 1-500 μm). Asshown in the figures, print head 14 may include a metal plate 26, aglass layer 28, and a microfabricated die 30. As will be described, eachof these components includes inlet and outlet sides and one or morefluid passages that interconnect the inlet and outlet sides of thecomponent in which they are formed. Each fluid passage is in fluidiccommunication with one or more ports, and each of the components are influidic communication with each other via one or more ports. Certainfluid passages may include one or more nozzles, and some embodiments offluid passages will be described below. A more detailed description ofillustrative print head components, including detailed nozzleconfigurations, dimensions, and methods of making a print head and itsvarious components may also be found in the U.S. Patent ApplicationPublications incorporated by reference above.

Print head 14 may be attached to fixture 10 so that print head 14 canreceive fluid from fixture 12 through outlet ports 18. In theillustrated embodiment, threaded fasteners 32 (only one is shown inFIG. 1) are included to removably fasten the print head 14 to thefixture 12. More specifically, the fasteners 32 pass through attachmentfeatures 34 formed in and through the metal plate 26 and further intoattachment features 20 formed in the mounting surface 16 of the fixture12. As used herein, the term “removable” and its derivatives, as used todescribe a type of attachment or fastener, indicates that the attachmentis intentionally configured so that the attachment can be undone orreversed without causing substantial damage or deformation of theattachment or attached components. Any non-removable attachment may bereferred to as a permanent attachment. Other suitable attachmentfeatures may be used to removably attach the print head 14 to thefixture 12, such as snap-fit features, press-fit features, clamps,clips, magnets, etc. In this case, attachment features 20, 32 arethreaded or unthreaded through-holes or apertures formed in theirrespective components and aligned to receive fasteners 32. As usedherein, any attachment or bond may be a direct attachment, where theattached components physically contact one another, or an indirectattachment, where the attached components may have at least portions ofone or more other components situated therebetween. For example, whenprint head 14 is assembled to the fixture 12, microfabricated die 30 isconsidered to be attached—more specifically, indirectly attached—to thefixture.

Referring now to FIG. 2, the print head 14 is shown removably fastenedto the fixture 12. One or more seals, such as O-ring seals 15 may beprovided at the interface of the metal plate 26 and mounting surface 16to prevent leakage of high temperature gases or other fluids from theprint head. In this particular example, where separate O-ring seals arelocated about each of the fluid outlet ports 18 (and the correspondinginlet ports of the metal plate 26), the seals serve not only to preventfluids from leaking from the print head, but also to prevent fluids fromleaking from each individual fluid source or thehead-to-fixture/port-to-port interfaces. In this example, the mountingsurface 16 further includes sealing features 25 (e.g., the counter-boresshown) formed therein to assist with locating the O-rings 15 duringassembly. Sealing features may additionally or alternatively be formedin a corresponding portion of the metal plate 26. In one embodiment, thesealing features may be raised features rather than recessed features,or the plate 26 may have features that fit into or around correspondingfeatures formed on or in the mounting surface 16. In some cases, thesealing feature may double as the seal where a suitable surface finish,material, and/or sealing contact area are available. In embodiments thatinclude compressible O-ring seals 15, fluoroelastomeric materials may besuitable due to their high temperature stability. Perfluoroelastomersare one type of fluoroelastomer that may be suitable for use at theplate-to-fixture interface. Kalrez perfluoroelastomer O-rings availablefrom the DuPont company may be suitable for use as seals 15. Non-organicmaterials may also be used to fabricate seals 15, preferably a ductilemetal such as aluminum or stainless steel, where the surfaces of theseals are of sufficiently high quality to form a fluid-tight seal withthe plate 26 and the fixture 12.

Referring now to FIGS. 1 and 2, microfabricated die 30 is shown attachedto metal plate 26. In this embodiment, the die 30 is indirectly attachedto the metal plate 26 via the glass layer 28 that is interposed betweenthe metal plate 26 and the die 30. In one embodiment, the directattachment at the interface formed between the metal plate 26 and theglass layer 28 is a permanent bond 36, and the direct attachment at theinterface formed between the glass layer 28 and the microfabricated die30 is a permanent bond 38. Direct, permanent bonds may be formed betweenadjacent components by anodic bonding, gold diffusion welding, transientliquid phase bonding, the application of certain adhesives, or othersuitable techniques that can also form a fluid-tight seal at the bond.In one embodiment, both of bonds 36 and 38 are anodic bonds and togetherform a double anodic bond that bonds opposite sides of the glass layer28 to the metal plate 26 and the die 30.

Metal plate 26 is a component to which microfabricated die 30 and/orglass layer 28 may be attached so that the die 30 can be easily removedfrom the fixture without the necessity of breaking a bond between thefixture 12 and the more fragile glass layer 28 or die 30. In otherwords, when removable print head 14 is separated from the fixture 12,metal plate 26 stays with the print head. In addition to earlierdescribed elements, metal plate 26 in the illustrated embodimentincludes inlet side 40, outlet side 42, and at least one fluid passage44. Inlet side 40 in this embodiment is the surface of the plate 26nearest the fixture 12, and outlet side 42 is the surface opposite theinlet side. Fluid passages 44 fluidly interconnect the inlet and outletsides of plate 26 so that fluid may flow therebetween and through theplate 26. Each fluid passage 44 interconnects a fluid inlet port 46 atthe inlet side 40 and a fluid outlet port 48 (FIG. 1) at the outlet side42. In this instance, each fluid passage 44 is a straight-throughpassage in an axial direction that is in alignment with the ports 46,48. As used herein, ports are openings that provide flow access to andfrom passages formed in a component from a location outside of thecomponent and are defined at the surface of the component in which thepassage is formed (i.e., the intersection of the passage and thecomponent surface). In some cases, such as with fluid passages 44, theports 46, 48 are substantially the same size and are aligned with theremainder of the passage. This may also be the case where an elongatedslot is formed completely through a plate or layer of the print head. Inother cases a port may provide access to a fluid passage formed in acomponent, where the fluid passage changes direction within thecomponent.

Metal plate 26 may be constructed from a variety of materials, includingiron-nickel-cobalt alloys, titanium, or other metal having asufficiently low and/or uniform coefficient of thermal expansion. Inparticular, alloys that are formulated to have thermal expansioncoefficients that are compatible with the thermal expansion coefficientof glass layer 28 are preferred. Suitable Fe—Ni—Co alloy for metal plate26 is available under the trade name Kovar. The metal used to constructthe plate 26 may be hot rolled and/or annealed to minimize and internalstress in the plate 26 for more uniform expansion and contraction duringuse. Metal plate 26 may range in thickness from about 1 mm to about 3 mmor higher. Where anodic bonding is used to join components to a surfaceof the metal plate 26, the joining surface (in this case, outlet side42) is preferably prepared for bonding by processing the surface so thatthe average or RMS surface roughness is about 20 nm or less, where lowersurface roughness is better. A combination of abrasive cleaning,progressively finer polishing and pickling may achieve a suitably lowsurface roughness for bonding.

Glass layer 28 is a layer interposed between the metal plate 26 and themicrofabricated die 30 and may serve as a thermal insulator between theplate 26 and die 30. In some embodiments, it may alternatively be knownas an insulator layer, a channel layer, or a fluid circuit layer todescribe one or more of its possible functions, and skilled artisans maydevise constructions that utilize materials other than glass. In theillustrated embodiment, glass layer 28 includes an inlet side 50, anoutlet side 52 (FIG. 1), and at least one fluid passage 54. Inlet side50 in this embodiment is the surface of the layer 28 nearest the fixture12, and outlet side 52 is the surface opposite the inlet side. Fluidpassages 54 fluidly interconnect the inlet and outlet sides of layer 28so that fluid may flow therebetween and through layer 28. Each fluidpassage 54 interconnects a fluid inlet port 56 at the inlet side 50 andat least one fluid outlet port 58 at the outlet side 52. In thisinstance, the fluid inlet ports 56 of the glass layer 28 are coincidentwith the fluid outlet ports 48 of the metal layer, as they are the samesize, are aligned with one another, and are formed through opposingsurfaces in intimate contact. This may not always be the case in allembodiments, however. Also in this example, fluid outlet ports 58 arenot defined by the glass layer alone, but by the interface of the inletports of die 30 with the open fluid passage 54 of layer 28. Referring toFIG. 2, fluid passage 54 is partially defined by a channel formed alongthe surface of outlet side 52 of the glass layer. When not assembledwith the die 30, passages 54 are not enclosed passages, rather they areopen along their length. Passages 54 appear as lines on outlet side 52of the glass layer 28 in FIG. 1. The ends of each visible portion ofpassages 54 represent the locations of inlet ports 56.

Referring again to FIGS. 1 and 2, FIG. 1 shows multiple fluid passages54 formed in the outlet side of the glass layer 28, while FIG. 2 is across-section through a single passage 54. The fluid passage 54 shown inFIG. 2 is an example of a mixing chamber, as it connects two fluid inletports that are in independent fluidic communication with different fluidsources, represented as the separate conduits 22, 22′ in FIG. 2. One ormore fluid passages 54 in the glass layer 28 may alternatively splitfluid flow from the same conduit 22 of fixture 12 (same inlet port ofthe glass layer) so that it feeds two or more separate inlet ports ofthe die 30. One or more fluid passages may also neither combine nordivide fluid flowing therethrough. Thus, a variety of fluid passage 54configurations may be used in the glass layer 28 to distribute and/ormix incoming fluids or allow incoming fluids to pass directly to theoutlet side 58 as desired. A plurality of separate fluid passages 54 mayat least partly define fluid circuits that determine how the incomingfluids will be processed prior to exiting the print head 14. In oneembodiment, a plurality of separate fluid passages 54 may be configuredso that a host material is mixed with a different dopant in each of theseparate passages. This may be useful for construction of a multi-colorOLED print head in which individual dopants selected for their effect onthe color of light emitted by the resulting OLEDs are individually mixedwith the same host material, or with different host materials, withinthe print head. In another example, one removable print head may includea fluid circuit that mixes an organic semiconductor host material withone type of dopant to pattern or print an OLED that emits a particularcolor of light. That particular print head may be removed and replacedwith a different print head that includes a fluid circuit that mixes thesame host material with a different type of dopant to pattern or printan OLED that emits a different color of light by fluidly connecting adifferent inlet port of the glass layer with the inlet port throughwhich the host material flows. It is also possible for similar mixingchambers or flow splitters to be formed in the inlet surface of the die30 or for complimentary features to be formed in both the inlet surfaceof the die 30 and the outlet side 52 of the glass layer 28. Glass layer28 may be constructed from borosilicate glass such as Pyrex or fromother types of thermally insulating glass materials to help isolate theheated fixture from the die 30. The glass preferably includes an ionicalkali metal oxide compound in solid solution to facilitate anodicbonding where desired. In one embodiment, the glass layer 28 is about500 μm thick, and the channels formed in the surface of the glass layermay be about 100-200 μm deep. Of course, these values are non-limitingand disclosed only for purposes of describing one particular embodimentand demonstration of the general size scale of microfluidic devices.

Microfabricated die 30 is a component that receives fluid and ultimatelydeposits or otherwise disperses the fluid onto or toward a substrate orother component. The term “microfabricated” refers to the dimensionalscale on which some of the features of the die are formed. Certainfeatures of die 30 may range in dimension from about 1 μm to about 500λm, with certain flow passage features having dimensions generally inthe 10 to 100 μm range. As with the other layers of print head 14,microfabricated die 30 includes an inlet side 60, an outlet side 62 andat least one flow passage 64 that fluidly interconnects the inlet andoutlet sides of the die 30. More specifically, flow passages 64 fluidlyinterconnect inlet ports 65 (coincident with outlet ports 58 in theglass layer 28) and outlet ports 66. Each of the flow passages 64 maycomprise a nozzle 68 having a reduced cross-section compared to itscorresponding inlet port 58. As already noted, some of the possiblenozzle configurations and methods of making a microfabricated dieincluding one or more nozzles are disclosed in finer detail in the U.S.Patent Application Publications earlier incorporated by reference. Aplurality of nozzles may be grouped together in a pattern across the die30 to define a nozzle array. Each of the nozzles may receive the same ordifferent mixture of vaporized organic materials, or some of the nozzlesmay receive the same first mixture of materials and others may receivedthe same second mixture of materials, all depending on the configurationof the fluid circuit. In one embodiment, the die 30 may be constructedfrom silicon or a silicon-based material (a material having silicon asits main constituent) and can be referred to as a silicon-based plate.Metal or ceramic materials may used as well, where suitable processingtechniques are available to accomplish microfabrication.

An organic vapor jet print head 14 or other microfluidic deviceconstructed as described above can allow for print head replacementwithout the need for physically breaking or damaging any of thecomponents, including the attachment itself, during replacement. It alsoreduces or eliminates the need to recondition the mounting surface ofthe fixture to which the print head is attached before attaching adifferent one. The above-described print head constructions may offerthe additional advantage of easy interchangeability so that the printhead can be removed and interchanged with another print head before ithas reached the end of its useful life, then reinstalled for further useat a later time. For example, print heads having different print linespacing, different nozzles shapes, different nozzle array patterns,and/or different fluid circuits that mix the same source materialsdifferently may be easily interchanged with one another. Additionally,the absence of organic adhesive materials at the various interfacesbetween components may be advantageous, as these types of materials canoutgas or vaporize at high temperatures and potentially contaminate thefluids flowing through the print head.

With reference to FIGS. 3 and 4, an illustrative method of forming adouble anodic bond will now be described, and this method may be used toconstruct the embodiment of FIGS. 1 and 2 or to construct one or moreother embodiments of microfluidic or other devices. According to oneembodiment, the method generally includes at least the step of applyinga voltage across a plate stack using positive and negative electrodes,wherein the negative electrode is in contact with a silicon-based plateof the plate stack. This method step runs counter to typical anodicbonding processes for silicon wafers, in which the positive electrodefrom the voltage source is normally placed in contact with the siliconwafer, not the negative electrode. In the particular embodimentdescribed below, the method includes providing a plate stack of threeplates such as in FIGS. 1 and 2, including a metal plate 126, glassplate 128, and silicon-based plate 130. Two of plates, such as the glassplate 128 and silicon-based plate 130 can be bonded together via ananodic bond formed in a suitable manner using voltage applied across theplate stack, with the third plate then being stacked on the remainingexposed side of the glass plate. A second anodic bond is then formed,this one between the third plate (e.g., metal plate 126) and the glassplate 128, by applying a voltage having a polarity that is the reverseof that used to form the first anodic bond. This method step may be partof a process having additional steps performed before and/or afterapplying the negative side of an electrical potential to thesilicon-based plate of the plate stack.

More particularly, as shown in FIG. 3, a glass plate 128 may be stackedtogether with a silicon-based plate 130 as shown so that surfaces ofeach plate oppose each other at an interface 138. A voltage V₁ may beapplied across the stacked plates with a positive electrode (or cathode)100 in contact with the silicon-based plate 130 and a negative electrode(or anode) 200 in contact with the glass plate 128. The applied voltagemay be at or around 1000V or between about 800V and about 1200V. Thehigher the voltage, the faster an anodic bond will form at theinterface. Other typical anodic bonding process parameters may becontrolled as well, such as the temperature, chamber pressure, and thepressure or clamp force applied to opposite sides of the stacked plates.A sufficiently strong anodic bond at interface 138 may be formed inabout 5-10 minutes at a voltage of about 1000 V.

As shown in FIG. 4, metal plate 126 may then be stacked together withthe anodically bonded glass and silicon-based plates 128, 130 so that asurface of the metal plate 126 opposes the exposed surface of the glassplate 128 at an interface 136. As described above, the surface of themetal plate 126 to be bonded with the glass plate 128 may be preparedfor bonding by ensuring that the surface roughness of the metal platebonding surface is sufficiently low. In addition, though typical anodicbonding processes do not require the surface of the glass plate 128 tobe prepared, some surface preparation may be necessary for the exposedsurface of the glass plate 128 after the anodic bond with thesilicon-based plate is formed. Due to ionic transport within the glassplate 128 during the anodic boding process, a surface layer or aprecipitate of an alkaline oxide, such as NaO, may be present at theexposed side of the glass plate 128. In one embodiment, an RCA cleaningprocess may be used to clean the exposed surface of the glass plateprior to placing the metal plate 126 thereon. Of course, other cleaningprocesses may be used.

FIGS. 5-7 illustrate exemplary metal plate 126 and glass plate 128surfaces that may be suitable for use in anodic bonding. FIG. 5 is anatomic force micrograph (AFM) of one surface of a metal plate that hasbeen prepared for anodic bonding. The particular metal plate surfaceshown in FIG. 5 was prepared by a process including the steps ofannealing, grinding, lapping, polishing, and acid pickling to achieve anRMS surface roughness of about 14 nm. FIG. 6 is an AFM of the exposedsurface of a glass plate after the opposite surface has been anodicallybonded to a silicon-based plate. The RMS surface roughness of theillustrated surface is about 3.0-3.5 nm, and the light spots shown alongthe surface are believed to be NaO precipitates. FIG. 7 is an AFM of theexposed surface of the glass plate of FIG. 7 after an RCA cleaningprocedure. The RMS surface roughness of the illustrated surface is about3.0-3.5 nm, or about the same as the roughness prior to the RCAcleaning, but the precipitates have been removed.

Referring again to FIG. 4, the plate stack including the bonded glassand silicon-based plates 128, 130 and the metal plate 126 is placedbetween electrodes 100 and 200 again, and a voltage V₂ is applied acrossthe stack to form an anodic bond at the interface 136, therebycompleting the formation of the double anodic bond. However, in thissecond anodic bonding step, the polarity of the voltage is reversed;i.e., the electrode that is in contact with the silicon based plate 130becomes the anode and the opposite electrode becomes the cathode.Because the anodic bonding process relies on bringing negative oxygenions to the bonding surface to covalently bond the glass surface to theopposite surface, the positive electrode must be positioned to attractthe negative ions toward the metal plate 126. A typical anodic bondingprocess does not expose a silicon-glass interface to current flow inthis opposite direction.

In addition to the polarity of the voltage being reversed to form thesecond anodic bond, V₂ may be lower than V₁ as well, in order to avoiddielectric breakdown of the silicon-based material and/or the partial orcomplete elimination of the anodic bond at interface 138. In oneembodiment, the voltage applied across the plate stack in the secondanodic bonding step may be no greater than about 800 V, and theinitially applied voltage V₂ may be even lower, such as about 500 V orless. The time required to form the second bond of the double anodicbond may thus be greater than the time required to form the first anodicbond. The time to form the second anodic bond may range from about 30 toabout 60 minutes, for example, at an applied voltage of about 700-800V.

In another embodiment, V₂ is applied as a variable voltage. For example,the initial value of V₂ may be in a range from about 400 V to about 600V, and the final value may range from about 800 V to about 1000 V. Therate of voltage increase during the formation of the second anodic bondmay range from about 10 to about 20 V/min so that from a starting V₂ of500V, about 25-50 minutes is required to reach 1000 V. V₂ may becontinuously increased, or it may be increased in discrete steps. Thisgradual application of V₂ to form the second anodic bond is thought toallow time for the uneven distribution of ions in the glass plate, dueto the first anodic bonding step, to at least partially equalize. Thevoltage and time combinations are non-limiting and may vary depending onthe particular materials and the size of the components being bonded.For example, the above ranges may be applicable for plates or layershaving a diameter or average width across their bond surfaces thatranges from about 15 mm to about 25 mm. Some embodiments include bondingplates having diameters or average widths ranging from about 50-100 mm,or about 75-80 mm in one particular implementation. The time required toform sufficient bonds with plates having larger dimensions may begreater due to the increased surface area of the bond surfaces and theassociated increase in the number of individual atomic bonds to beformed.

Forming a double anodic bond at opposite sides of a glass plate, such asglass layer 38 described in the microfluidic device embodimentspresented above, allows for superior seals at the interfaces between thevarious layers of the device. Anodic bonds can withstand the highoperation temperatures associated with OVJP, typically around 300° C.,with little to no effect on bond strength. Additionally, anodic bondingmay offer advantages over polymer or other organic-basedsealant/adhesive materials. Most organic materials will begin tobreakdown or degrade at such high operating temperatures. Even polymericmaterials that are rated for use at high temperatures and that maygenerally maintain chemical stability at high temperatures may tend tooutgas—i.e., low molecular weight substances, additives, or residualunreacted monomer may be forced out of the material. In an OVJPapplication, this may be exceptionally detrimental because the outgassedorganics may mix with the vapors that are intended to be deposited on asubstrate, thereby contaminating the desired organic materials anddecreasing performance of the resulting opto-electronic devices.

It is to be understood that the foregoing description is of one or moreembodiments of the invention. The invention is not limited to theparticular embodiment(s) disclosed herein, but rather is defined solelyby the claims below. Furthermore, the statements contained in theforegoing description relate to the disclosed embodiment(s) and are notto be construed as limitations on the scope of the invention or on thedefinition of terms used in the claims, except where a term or phrase isexpressly defined above. Various other embodiments and various changesand modifications to the disclosed embodiment(s) will become apparent tothose skilled in the art.

As used in this specification and claims, the terms “e.g.,” “forexample,” “for instance,” “such as,” and “like,” and the verbs“comprising,” “having,” “including,” and their other verb forms, whenused in conjunction with a listing of one or more components or otheritems, are each to be construed as open-ended, meaning that that thelisting is not to be considered as excluding other, additionalcomponents or items. Other terms are to be construed using theirbroadest reasonable meaning unless they are used in a context thatrequires a different interpretation.

1. A microfluidic device for receiving fluids from a fixture,comprising: a metal plate having a surface that includes a fluid outletport; a glass layer having an inlet side and an opposite outlet side andbeing bonded on the inlet side to the surface of the metal plate, theglass layer having a fluid passage interconnecting the inlet and outletsides of the glass layer to allow fluid flow through the glass layer, atleast a portion of the fluid passage being aligned with at least aportion of the fluid outlet port of the metal plate, whereby pressurizedfluid exiting the fluid outlet port of the metal plate enters the fluidpassage and is communicated to the outlet side of the glass layer; and amicrofabricated die having an inlet side and being bonded on the inletside to the outlet side of the glass layer, the die having a fluidpassage to allow fluid flow through the die, at least a portion of thefluid passage of the die being aligned with at least a portion of thefluid passage of the glass layer, whereby the pressurized fluidcommunicated to the outlet side of the glass layer enters the fluidpassage of the die; wherein the glass layer is directly bonded to themetal plate via an anodic bond.
 2. The microfluidic device set forth inclaim 1, wherein the metal plate further comprises: a fluid inlet port;a fluid passage interconnecting the inlet and outlet ports of the metalplate; and a sealing surface about the fluid inlet port of the metalplate.
 3. The microfluidic device set forth in claim 2, furthercomprising: an attachment feature corresponding to a fixture attachmentfeature, the features being arranged so that the inlet port of the metalplate aligns with an outlet port of the fixture, and so that a sealdisposed about the outlet port of the fixture engages the sealingsurface of the metal plate when the attachment features are engaged toattach the device to the fixture.
 4. The microfluidic device set forthin claim 1, further comprising a mixing chamber formed at least in partby one of said fluid passages.
 5. The microfluidic device set forth inclaim 1, wherein the metal plate includes an additional outlet portaligned with a different portion of the fluid passage of the glass layerso that the fluid passage of the glass layer interconnects both of theoutlet ports of the metal plate and conducts fluid received from theoutlet ports of the metal plate to the outlet side of the glass layer.6. The microfluidic device set forth in claim 1, wherein the glass layerfurther comprises: a plurality of separate fluid passagesinterconnecting the inlet side and the outlet side of the glass layer,wherein each of the separate fluid passages interconnects a fluid inletport at the inlet side of the glass layer and a fluid outlet port at theoutlet side of the glass layer; wherein at least one of the separatefluid passages is in fluidic communication with an additional fluidinlet port at the inlet side of the glass layer or an additional fluidoutlet port at the outlet side of the glass layer.
 7. The microfluidicdevice set forth in claim 1, wherein the microfabricated die furthercomprises an outlet side, and the fluid passage of the microfabricateddie further comprises: a fluid inlet port at the inlet side of the die;a fluid outlet port at the outlet side of the die; and a nozzle locatedbetween and in fluidic communication with the fluid inlet port and thefluid outlet port of the die.
 8. The microfluidic device set forth inclaim 1, wherein the microfabricated die further comprises an outletside, and the device further comprises: a plurality of outlet portsformed in the metal plate; and a plurality of nozzles formed in themicrofabricated die such that each nozzle provides a spray of fluid viaan associated outlet port at the outlet side of the die, wherein eachone of the nozzles fluidly connects its associated outlet port with atleast one of the outlet ports of the metal plate via the glass layer. 9.The microfluidic device set forth in claim 8, wherein at least one ofthe nozzles is fluidly connected to a different one of the outlet portsof the metal plate than another one of the nozzles.
 10. The microfluidicdevice set forth in claim 1, wherein the glass layer is directly bondedto the die via an anodic bond, whereby the microfluidic device includesa double anodic bond.
 11. An organic vapor jet printing device,comprising: a fixture having fluid outlet ports for supplying hightemperature gases under pressure; a metal plate fastened to the fixture,the metal plate having fluid inlet ports, each aligned with one of thefluid outlet ports of the fixture to receive the high temperature gases;a seal located at an interface of the fixture outlet ports and plateinlet ports to prevent leakage of the high temperature gases out of theprinting device; and a silicon-based die bonded to the metal plate andhaving fluid inlet ports and nozzles that are in fluidic communicationwith the fluid inlet ports of the die, the metal plate having fluidoutlet ports that are in fluidic communication with the inlet ports ofthe metal plate and that are in fluidic communication with the fluidinlet ports of the die, whereby the high temperature gases are conductedfrom the fixture, through the seal, through the metal plate and tonozzles of the silicon-based die.
 12. The printing device set forth inclaim 11, wherein the metal plate is removably fastened to the fixtureand wherein the silicon-based die is permanently bonded to said metalplate.
 13. The printing device set forth in claim 11, further comprisinga glass layer interposed between the die and metal plate, wherein themetal plate is bonded to the die via the glass layer, and wherein theglass layer is directly bonded to the metal plate and die via a doubleanodic bond.
 14. The printing device set forth in claim 11, wherein aseparate seal surrounds each fluid inlet port of the metal plate and theseal includes a fluoroelastomer material.
 15. A method of forming adouble anodic bond, comprising the steps of: (a) stacking a glass platetogether with one of: a silicon-based plate and a metal plate, so thatplanar surfaces of each plate contact each other at an interface; (b)forming an anodic bond at the interface by applying a voltage across thestacked plates; (c) stacking the other one of the silicon-based plateand the metal plate together with the bonded plates so that the glassplate is interposed between the silicon-based plate and the metal plate;and (d) applying a voltage across the stacked metal, glass, andsilicon-based plates to form the double anodic bond, wherein thepolarity of the voltage is reversed from the voltage applied in step (b)with respect to the plates bonded in step (b).
 16. The method of claim15, wherein the anodic bond of step (b) is formed between the glassplate and the silicon-based plate, and step (d) includes placing anegative electrode in contact with the silicon-based plate.
 17. Themethod of claim 15, further comprising the step of: cleaning the surfaceof the glass plate opposite the bonded surface using an RCA cleaningprocess before step (c).
 18. The method of claim 15, further comprisingthe step of: preparing a bonding surface of the metal plate so thatsurface roughness of the bonding surface is about 20 nm or less beforebonding the metal plate with the glass plate.
 19. The method of claim15, wherein the voltage applied in step (d) is lower than the voltageapplied in step (b).
 20. The method of claim 15, wherein step (d)includes gradually increasing the applied voltage.